Functional significance of the conserved C-Terminal VFVNFA motif in the retina-specific ABC transporter, ABCA4, and its role in inherited visual disease.

The human retina-specific ATP binding cassette transporter, ABCA4, plays a significant role in the visual cycle. Mutations in the ABCA4 gene result in a broad spectrum of severe, blinding, retinal degenerative diseases, including Stargardt macular dystrophy, fundus flavimaculatus, autosomal recessive (ar)-retinitis pigmentosa, and ar-cone-rod dystrophy. Genetic testing frequently yields novel variants of unknown significance, making accurate prognosis and therapeutic approaches difficult. Recently, we have reported a novel variant of ABCA4 corresponding to a four-nucleotide deletion which led to a premature stop codon and loss of the last 161 amino acids, including the highly-conserved VFVNFA motif. Despite the presence of this motif among other ABCA proteins, knowledge of the functional significance of this sequence remains limited. In this study, we have conducted structural and functional analyses of recombinant ABCA4 polypeptides with altered VFVNFA motifs to evaluate the importance of this sequence. Further investigation of ABCA4 subdomain interactions, using Fluorescence Resonance Energy Transfer, demonstrated a loss of interaction between nucleotide binding domains in the absence of the VFVNFA motif.